The present invention relates to treatment fluids useful in subterranean applications and more particularly, to crosslinkable-polymer compositions that comprise an aqueous fluid, chitosan, a chitosan-reacting polymer, and a gelation-retarding additive comprising an acid derivative, and associated methods.
At some point in the life of a well, it may be desirable to mitigate the flow of fluids through a portion of a subterranean formation that is penetrated by the well. In some instances, it may be desirable to control the flow of fluids introduced into the well so that the flow of the fluid into high-permeability portions of the formation is mitigated. For example, in an injection well, it may be desirable to seal off high-permeability portions of a subterranean formation that would otherwise accept most of an injected treatment fluid. By sealing off the high-permeability portions of the subterranean formation, the injected treatment fluid may thus penetrate less permeable portions of the subterranean formation.
In other instances, it may be desirable to mitigate the production of undesired fluids (e.g., water) from the well. The production of water with oil and gas from wells constitutes a major problem and expense in the production of oil and gas. While oil and gas wells are usually completed in hydrocarbon-producing formations, a water-bearing zone may occasionally be adjacent to the hydrocarbon-producing formation. In some instances, the higher mobility of the water may allow it to flow into the hydrocarbon-producing formation by way of, among other things, natural fractures and high-permeability streaks. In some circumstances, the ratio of water to hydrocarbons recovered may, over time, become sufficiently high that the cost of producing, separating, and disposing of the water may represent a significant economic loss.
One attempt to mitigate the flow of fluids through a portion of a subterranean formation has been to place crosslinkable-polymer compositions in a well bore so as to cause them to enter the portion of the subterranean formation such that they may crosslink therein. The crosslinking of these compositions tends to produce crosslinked gels, which may eliminate, or at least reduce, the flow of water or other undesirable fluids through the natural fractures and high-permeability streaks in the formations. One particular crosslinkable-polymer composition involves the use of chitosan to crosslink a water-soluble polymer, such as an acrylamide-based polymer.
The time required for a crosslinkable-polymer composition to form the desired crosslinked gel can vary widely. This length of time, sometimes referred to as “gelation time,” varies, depending on a number of factors, including the type of crosslinking agent used, the type of polymer used, the type of aqueous fluid used, concentrations of components used, the pH, the temperature, and a variety of other factors. Delaying the gelation of a crosslinkable-polymer composition may be desirable to allow, among other things, pumping of the composition to its desired location. The desired gelation time varies depending on the specific application. For instance, for wells of considerable depth or increased temperature, a longer gelation time may be required to deliver the crosslinkable-polymer composition to its desired destination before the composition forms the crosslinked gel.
In subterranean formations, a wide range of temperatures may be encountered presenting challenges to the use of crosslinkable-polymer compositions therein. For example, if the temperature of the subterranean formation is sufficiently high, the crosslinkable-polymer composition may gel prematurely. To counteract this undesirable possibility, oftentimes, the crosslinkable-polymer composition must be designed such that its gelation time is delayed or retarded. That is, the thickening and gelation characteristics of the crosslinkable-polymer composition must be altered such that the time it takes the crosslinkable-polymer composition to form a crosslinked gel is delayed for an amount of time sufficient to permit the crosslinkable-polymer composition to be pumped to its desired destination.
A number of methods for adjusting the gelation time of crosslinkable-polymer compositions have heretofore been used. For instance, the gelation time of the above mentioned crosslinkable-polymer compositions, that comprise chitosan and a water-soluble polymer, may be lengthened by increasing the level of bulky and/or less reactive monomers in the selected polymer or polymers used. These modifications, however, may still be inadequate to provide the desired gelation times for certain applications.